Device for detecting grounds in electric furnaces



Feb. 8, 1966 F. R. MATH WS 3,234,316

DEVICE FOR DETECTING ROUNDS IN ELECTRIC FURNACES Filed March 20, 1963 g\Q q N O N O JG) 0) J D g2 0 Lu; f

INVENTOR FRANK ROBERT MATHEWS TTORNEY United States Patent C) 3,234,316DEVICE FOR DETECTING GROUNDS IN ELECTRIC FURNACES Frank Robert Mathews,Salem, Ohio, assignor to The Electric Furnace Company, Salem, Ohio, acorporation of Ohio Filed Mar. 20, 1963, Ser. No. 266,694 13 Claims.(CI. 13-24) The present invention is directed toward the art ofelectrical furnaces and more particularly to a device for detecting agrounded heating element in an electrical resistance furnace.

An electrical resistance furnace of the type having a metal frame andone or more electrical heating elements, has become common equipment inindustry for various heating tasks. Such furnaces are well adapted forbatch heat treating metal articles, for heating articles carried on ametal conveyor traveling through the furnace and for heating anelongated workpiece, such as longitudinal- 'ly moving metal tubing, asit passes through the furnace. The heating element in such a furnace isusually exposed within the interior of the furnace because anyprotective structure surrounding the element would form a heat barrierwhich would decrease the heating efficiency of the furnace.

-With the heating elements being unprotected, it is comrnon knowledgethat the elements are often accidentally grounded to the frame of thefurnace. For instance, in heat treating continuously moving tubing, thetubing often strays from its intended path a sufiic-ient amount to allowcontact between the grounded workpiece and the exposed heating element.Further, when an operator is loading or unloading a workpiece from abatch heating furnace, the

tool with which he grips the workpiece, such as a pair of tongs, oftencontacts the electrical element. In some cases, from abuse of thefurnace or careless charging of a workpiece into a furnace, a portion ofthe furnace may actually be bent into contact with the heating element.Also, accumulation of conductive scrap particles in the furnace or theaccumulation of deposits from various atmospheres that may be used inthe furnace can cause a conductive path between the heating element andthe frame of the furnace.

Such accidental grounds or connections between the heating element andthe frame of the furnace, may cause arcing which can seriously damagethe heating element, this is especially true if the heating element isgrounded at two widely spaced points. In addition, if the groundconnection is through a workpiece within the furnace, and the ground isallowed to continue for a substantial time, the workpiece may actuallybe welded onto the heating element so that removal of the workpiece fromthe furnace will cause substantial damage to the heating element.

The present invention is directed toward a device for detecting anaccidental ground or other connection between the heating element andthe frame of an electric furnace which device is responsive to groundingof the element at only one point and before any damage to the heatingelement or injury to an operator can occur.

In accordance with the present invention there is provided animprovement in the combination of a furnace having a metal frame, anelectrically powered heating element adjacent the frame, and switchmeans for deenergizing the heating element. This improvement comprises,connecting the power supply in a series circuit extending from the frameto the element, a signal device, and actuator means in the seriescircuit and responsive to the flow of current through the circuit forenergizing the signal device.

In accordance with a more limited aspect of the present invention thereis provided the improvement as defined above wherein the signal deviceincludes 21 normally closed switch and means responsive to opening ofthe switch for actuating the switching means to deenergize the heatingelement, and the actuator means as defined above includes a means foropening this switch when current is flowing in the series circuit.

The primary object of the present invention is the provision of a devicefor detecting accidental grounds in the heating element of a resistanceheating furnace.

Another object of the present invention is the provision of a device fordetecting an accidental ground in the heating element of an electricfurnace which device is responsive to a one-point ground on the element,which usually is not in itself dangerous, instead of a twopoint groundwhich will usually cause an immediate, harmful reaction, such as arcingat the element or shock to an operator.

Another object of the present invention is the provision of such aground detecting device which is durable in operation, inexpensive toapply to existing equipment and easy to repair.

Still another object of the present invention is the provision of adevice for detecting accidental grounds in the heating element in aresistance heating furnace which device includes a power supplyconnected in a series circuit between the heating element and thegrounded frame of the furnace and means responsive to the current inthis circuit for indicating an accidental ground.

' These and other objects and advantages will become apparent from thefollowing description used to illustrate the preferred embodiment of theinvention as read in connection with the accompanying drawing in whichthe preferred embodiment of the present invention is illustrated.

Referring now to the drawing, wherein the showing is for the purpose ofillustrating a preferred embodiment of the invention only and not forthe purpose of limiting same, there is shown, somewhat schematically, aresistance heating element A which is secured by insulated hangers ontoframe B of an electrically heated furnace and which is spaced onlyslightly from the frame B. The heating element and frame are graphicallyillustrated as being somewhat separated; however, it is appreciated thatthe frame B may be quite close to the resistance heating element in theactual construction of a heating furnace.

To energize the heating element A so that the element can createsufiicient heat within the furnace to accomplish a given heatingoperation, there is provided an AC. power supply 10 which, in accordancewith the preferred embodiment of the present invention, is a 440 volt,60 cycle line source that feeds a power transformer 12 having secondaryleads 14, 16 adapted to be connected onto heating element leads 18, 20by a double pole, single throw solenoid operated switch 22. Although avariety of switches could be provided for connecting leads 14, 16 withelement leads 18, 20, in accordance with the illustrated embodiment ofthe present invention, switch 22 comprises normally open contacts 24, 26controlled by an appropriate solenoid coil 28. In operation,energization of coil 28 closes contact members 24, 26 which connectsresistance heating element A across secondary leads 14, 16.

To control the operation of solenoid coil 28, there is provided aswitching circuit 30 comprising a line power supply 32, leads 34, 36 andnormally open switch 38. When switch 38 is closed, current is suppliedto coil 28 which closes switch 22, that connects element A across leads14, 16 as is common in the art of electric furnaces.

To interrupt switching circuit 30, there is provided, in lead 36, adisconnecting switch 40 which may be alternately actuated between anopen and closed position. Of course, switch 40 must be closed before thepower supply 32 can operate coil 28.

To control switch 38, in accordance with the illustrated embodiment ofthe present invention, there is provided a temperature control 50 whichmay be operated by a thermostat or other temperature responsive meanswithin the heating furnace so that a signal is generated by this control50 when further heating of the furnace is required. Temperature control50 is of a well known type to those skilled in the art and closes acontact to apply a control signal to coil 58 when there is need for heatto be supplied by heating element A. Power to operate coil 58 is takenfrom an internal alternating voltage supply source. After the furnacereaches a desired temperature, the control50 ceases to create a controlsignal. To direct the control signal from temperature control 50, thereis provided a temperature control circuit 52 having leads 54, 56connected across 'a solenoid coil 58. Further, in circuit 52 there isprovided a normally closed contact 60. As long as a signal is beinggenerated by control 50, i.e. the furnace requires further heating, coil58 is actuated to hold switch 38 in its closed position. In thisposition, coil 28 closes contacts 24, 26 when switch 40 is closed.

In accordance with the illustrated embodiment of the present invention,circuit 52 is controlled by the switch 60 which operates in response toa device 70 for detecting an accidental ground of the resistance'heatingelement A onto the frame B. For purposes of illustration, the accidentalground is represented by dotted line C and is defined as a lowresistance electrical connection between the element A and frame B. Sucha connection can result from deforming the element A or frame B intocontact with each other or by bridging between element A and frame Bwith an electrically conductive material.

In accordance with the preferred embodiment of the present invention,device 70 includes a series circuit from element A to frame B whichcircuit includes leads 72, 74. Within the series circuit there isprovided a power supply 76 which may take a variety of structuralembodiments; however, in accordance with the preferred embodiment of thepresent invention, the power supply includes a line source 78 coupled totransformer 80 which introduces an A.C. signal to leads 82, 84 forpowering rectifier 86. The output leads 88, 90 are connected to leads72, 74 to complete the series circuit between the element and thegrounded frame. Accordingly, a DC. potential exists between leads 72,74.

Within the output leads 88, there is provided a coil 92 and a holdingcapacitor 94 in parallel with the coil to prevent chatter of the coilwith fluctuations in the output of this rectifier so that the coil 92can control normally open switch 96. V

In operation of device 70 as so far explained, the accidental ground Celectrically connects leads 72 to lead 74, irrespective of the positionon element A at which the ground occurs. When these leads are connected,a current flows through the series circuit which energizes coil 92 andcloses switch 96. When the switch 96 is closed, switch 60 issimultaneously opened. Consequently, coil' 58 is de-energized, switch 38is accordingly opened, and coil 28 is de-energized. In this manner,switches 24, 26 are opened in response to an accidental ground betweenthe heating element and the furnace frame. It is noted that the ground Cis at only one point on element A; therefore, the ground does not causean A.C. current leakage from the element to the frame unless the elementor its power supply circuit is grounded to the frame in anotherlocation. Thus, with only one-point ground no damage is caused and thedevice 70 can operate before a further ground causes a dangerouscondition.

, To prevent coil 92 from dropping out with fluctuations in the currentflowing through the series circuit, i.e., be-

tween leads 72 and 74, there is provided a holding circuit or bypasscircuit 100 which circuit is closed by switch 96 so that current flowsthrough the holding circuit 100 between leads 88, irrespective ofvariations in the accidental ground C. In this manner, as soon as thedetrimental ground occurs the holding circuit maintains the coil 92energized so that the power cannot be re-applied to the resistanceheating element A.

Following correction of the accidental ground C, a reset switch 102 isprovided for interrupting the holding circuit 100. This allows dropoutof coil 92 so that the heating element A can again be re-energized bysource 10.

To notify an operator that the furnace has been shut down by the device70, there is included within the holding circuit a device 104 which, inthe preferred embodiment of the present invention, comprises a hell orlamp actuated by various arrangements in response to the flow of currentin the holding circuit. It is appreciated that another warning devicedifferent from a hell or lamp could be used in device 104. The device104 may be the only indicator of the ground C and the operator in suchan instance could manually open switch 22 when the device 104 isactuated to signal a ground.

In operation of the invention as shown in the drawing, the accidentalground closes switch 96 and opens switch 60 so that the element A isshut down and the holding circuit 100 is energized. Thereafter, theground may be corrected and reset switch 102 pushed to drop out theholding circuit 100 and prepares the device 70 for detection of futuregrounds of the heating element A.

In the embodiment described, the furnace heating element is energized byalternating current at a usual frequency such as 60 cycles. Theground-detecting current is superimposed on the main alternating currentin some part of the conducting path which includes the element. Usingdirect current for ground detection, suitable apparatus is provided torespond to the detecting direct current flow only, independent of themain alternating current fiow.

It the main power supply were direct current, an alternating currentcould be superimposed on it fcr detection purposes. Also, for an A.C.power circuit at normal frequency, it is possible to superimpose a highfrequency A.C. detecting current upon it and to provide apparatus whichresponds to the latter, independent of the standard frequency flow inthe main circuit. Consequently, the series circuit provides a detectioncurrent supply dissimilar for the current in the heating element and ameans for detecting the flow of this detection current.

The present invention has been discussed in connection with onepreferred embodiment; however, it is appreciated that various structuralchanges may be made without departing from the intended spirit and scopeof the invention as defined by the appended claims.

Having thus described my invention, I claim:

1. In the combination of a furnace having a metal frame, an electricallypowered heating element adjacent said frame, and switch means forde-energizing said heating element, the improvement comprising: meansfor detecting whether said element is grounded to said frame andincluding a power supply connected in a series circuit extending fromsaid frame to said element, signal means, and actuator means in saidseries circuit responsive to current flow through the circuit forenergizing said signal means.

2. The improvement as defined in claim 1 wherein said signal meanscomprises a switch and means responsive to actuation of said switch foractuating said switch means to de-energize said element, and saidactuator means comprising a means for actuating said switch when currentis flowing in said circuit.

3. The improvement as defined in claim 2 wherein said actuator meanscomprises a solenoid coil connected in said series circuit, and meansoperable by said coil and operably connected onto said switch.

4. The improvement as defined in claim 3 including a temperatureresponsive control circuit, said control circuit having means forde-energizing said element when said control circuit is open, saidswitch being a normally closed switch in series with said controlcircuit, and energization of said solenoid coil opening said switch.

5. The improvement as defined in claim 4 including a holding circuit forsaid coil, said holding circuit being in parallel with said coil andsaid power supply.

6. The improvement as defined in claim 5 including an electricallyenergized alarm and a means in said holding circuit for actuating saidalarm.

7. The improvement as defined in claim 6 including a manually operatedcircuit breaker in said holding circuit.

8. The improvement as defined in claim 1 wherein said actuator meanscomprises a solenoid coil connected in said series circuit, and meansoperable by said coil and operably connected onto said signal means forenergizing said signal means when said coil is energized.

9. The improvement as defined in claim 1 including a temperatureresponsive control circuit, said control circuit having means forde-energizing said element when said control circuit is open, saidsignal means including a normally closed switch in series with saidcontrol circuit, and said actuator means including means for openingsaid switch in response to a flow of current through said seriescircuit.

10. The improvement as defined in claim 9 wherein said actuator means isa solenoid operably connected onto said normally closed switch andincluding a holding circuit for said coil, said holding circuit being inseries with said coil and said power supply.

11. The improvement as defined in claim 1 wherein said signal means isan electrically energized alarm.

12. In the combination of a furnace having a metal frame, anelectrically powered heating element adjacent said frame, and switchmeans for de-energizing said heating element, the improvementcomprising: means for detecting whether said element is grounded to saidframe and including a first power supply connected across said heatingelement for energizing said element, a second power supply connected ina series circuit extending from said frame to said element, signalmeans, said second power supply having a detectably different electriccharacteristic than said first power supply, and actuator means in saidseries circuit responsive to flow of said detectably different currentfor energizing said signal means.

13. The improvement as defined in claim 12 wherein said first powersupply takes the form of an alternating current voltage supply and saidsecond power supply takes the form of a direct current voltage supply.

References Cited by the Examiner UNITED STATES PATENTS 2,196,362 4/1940Osborn 31718 2,400,749 5/ 1946 Foster 31718 2,542,838 2/1948 Reagan317-18 X 2,637,843 5/ 1953 Kammerdiener 317-18 X 3,086,999 4/1963 Kramer13--24 FOREIGN PATENTS 580,787 9/1946 Great Britain.

RICHARD M. WOOD, Primary Examiner,

1. IN THE COMBINATION OF A FURNACE HAVING A METAL FRAME, AN ELECTRICALPOWERED HEATING ELEMENT ADJACENT SAID FRAME, AND SWITCH MEANS FORDE-ERNERGIZING SAID HEATING ELEMENT, THE IMPROVEMENT COMPRISING: MEANSFOR DETECTING WHETHER SAID ELEMENT IS GROUNDED TO SAID FRAME ANDINCLUDING A POWER SUPPLY CONNECTED IN SERIES CIRCUIT EXTENDING FROM SAIDFRAME TO SAID ELEMENT, SIGNAL MEANS, AND ACTUATOR MEANS IN SAID SERIESCIRCUIT RESPONSIVE TO CURRENT FLOW THROUGH THE CIRCUIT FOR ENERGIZINGSAID SIGNAL MEANS.